Non-volatile semiconductor memory cells using a floating gate to store charges thereon and memory arrays of such non-volatile memory cells formed in a semiconductor substrate are well known in the art. Typically, such floating gate memory cells have been of the split gate type, or stacked gate type, or a combination thereof.
One of the problems facing the manufacturability of semiconductor floating gate memory cell arrays has been the alignment of the various components such as source, drain, control gate, and floating gate. As the design rule of integration of semiconductor processing decreases, reducing the smallest lithographic feature, the need for precise alignment becomes more critical. Alignment of various parts also determines the yield of the manufacturing of the semiconductor products.
Self-alignment is well known in the art. Self-alignment refers to the act of processing one or more steps involving one or more materials such that the features are automatically aligned with respect to one another in that step processing. Accordingly, self alignment minimizes the number of masking steps necessary to form memory cell structures, and enhances the ability to scale such structures down to smaller dimensions.
In the manufacture of memory cell arrays, it is also known to form cell elements that extend across the entire array of memory cells. For example, with an array having interlaced columns of isolation and active regions, with a plurality of memory cells in each active region, memory cell elements such as control gates, source regions, drain regions etc. can be formed to continuously extend across an entire row or column of memory cells. In order to ensure an equalized voltage on such elements for all the memory cells in the target row/column, strap regions have been used to provide multiple electrical connections along the length of continuously formed memory cell elements, so that uniform voltages are applied to all the memory cells in the affected row/column.
FIG. 1 illustrates a known strap region design. Strap region 1 is formed along side a memory cell array 2. The memory cell array 2 includes columns of active regions 3 interlaced with columns of isolation regions 4. Rows of memory cell pairs 5 are formed with word lines 6 and source lines 7 extending along the memory cell rows, with each pair of memory cells having two word lines 6 and sharing a single source line 7. (Those of skill in the art will recognize that the term source and drain may be interchanged. Further, the word line is connected to the control gate of the floating gate memory cell. Thus, the term control gate or control gate line may also be used interchangeably with the term word line). Typically, the word line and the source lines are made of polysilicon or polysilicide or salicide material. Thus, pure metal lines are used to strap these lines. Strap cells 8 are formed on the control gates 6 and source lines 7 as they traverse the strap region 1. Electrical contacts 9a and 9b are then formed onto the control gate (word) lines 6 and source lines 7 respectively by metal lines (not shown) traversing in the word line direction positioned above the array shown in FIG. 1 and electrically insulated therefrom for supplying the desired voltages to the various rows of control gates 6 and source lines 7.
Ideally, for larger memory arrays, a plurality of strap regions are interlaced within the memory cell array (e.g. one strap region for every 128 cells in the word line direction). Preferably, the strap regions are formed simultaneously with the process steps used to make the memory cell array.
As device geometries get smaller, it is increasingly difficult to reliably form electrical connections to the strap regions 8. The word lines 6 are very close to the source lines 7, and get even closer with smaller device geometries. As the distance between the control gate lines 6 and source line 7 shrinks, it becomes more difficult to form contacts 9a and 9b properly. For example, just a small shift of one of the control gate line 6 contacts toward an adjacent source line 7 would result in the contact being formed over both a word line 6 and a source line 7, thus shorting the two together. Further, there is simply no room to enlarge and separate the strap cells to increase the tolerance of the contact formation steps.
One or more logic or peripheral regions are also formed on the substrate as the memory cells and strap regions are formed. Peripheral regions are typically formed adjacent to the memory cell array on the same silicon substrate. Logic devices (i.e. MOS FET's, etc.) are formed in these regions to operate the memory cell array or perform logic functions related to the memory cell array. In order to form such logic devices along side the memory cell array, the memory cells, the logic devices and the strap regions are formed using some of the same processing steps. For example, certain elements (e.g. poly gates) of the logic devices and memory cells are often formed with the same processing steps, thus coupling the formation of these elements together. This can make it difficult to optimize elements of the logic devices without adversely affecting elements of the memory cells, and vice versa.
For multi-level cell designs, the source-line resistance plays a crucial role on programming a selected bit cell, in terms of stored charges, to a desired level. Low source-line resistance is often dictated by the required array efficiency as well as by the design tolerance for the multi-level cell. One way to reduce source-line resistance is to interlace a greater number of strap cells spaced closer together among the array of memory cells. However, the more strap cells used, the less space on the substrate that is available for memory cells, and thus the greater chip size needed to contain any given number of memory cells. The need for lower source-line resistance become even more acute as the total number of levels in the multi-level cell design is increased. Another way of reducing source-line resistance is by forming conductive polycide on the source-line poly using silicide technology that forms silicide on other regions (such as the source and drain). However, as the source lines are scaled down near the 0.1 μm regime, advanced suicide technologies of choice (such as Cobalt silicide) are not always available as a module for full process integration (e.g. due to FAB constraints). Even with CoSi, the Rsheet is typically 4 ohms/square, which is far above the 1 ohms/square often deemed necessary for multi-level cell designs.
Thus, there is a need for a manufacturing method that efficiently forms the memory cells, the logic devices and the strap cells using the same processing steps, where these devices can be scaled down to very small device geometries and still provide a low source line resistance.